This can be a pain to do. You have to detach your arms using either RB or LB from your body with guns in them. Using the SMGs, Assault Rifle or the Bowgun will give you the best results. Make sure there are no Grand-babies in the room when you try this as they will suck in your arm and eat it. The early chapters have small rooms & dont have Grand-babies in them. Best level you can do this is the Asylum courtyards.